This invention relates to gas turbine engines and, more particularly, to reducing turbomachinery noise generated therein.
The invention herein described was made in the course of, or under, a contract (or grant) with the United States Department of Transportation.
With increased public concern over aircraft-generated noise, the aircraft gas turbine engine manufacturer is faced with the problem of developing new ways of effectively reducing noise. One of the noise sources is that associated with the viscous shearing between rapidly moving exhaust gases and the surrounding atmosphere, a phenomenon with which the present invention is not concerned. It is the noise generated by the turbomachinery within the gas turbine engine to which the present invention is directed. This turbomachinery noise results from the relative motion of adjacent blade rows, typical of those found in compressors (including fans) and turbines. For example, a compressor comprises multiple bladed stages, each stage including a rotatable blade row and a stationary blade row. It has long been recognized that in axial flow turbomachines one of the principal noise sources is the interaction between the wakes of stationary upstream blade rows and moving downstream blade rows (rotors). This so-called "viscous wake interaction" results in noise at the rotor blade passing frequency and at its harmonics.
One of the early methods employed to reduce this viscous wake interaction noise was to increase the axial spacing between the rotating and stationary blade rows. This modification provided space for the viscous wake to dissipate before reaching the downstream blade row, resulting in less blade passing frequency and harmonic noise.
Increased spacing can generally be applied to turbomachines with one or possibly two stages without seriously affecting the performance of the machine. However, if axial space is at a premium or if more than two stages are involved, increases in axial length of the machine may be restricted by weight, aerodynamic performance losses, cost and/or installation and space requirements. Thus, another means of reducing the viscous wake interaction effect is desirable.